Many fuel system problems in mechanical equipment stem from the residual fuel that is left in the carburetor during periods of long storage. In some cases, the fuel polymerizes and forms a hard substance commonly referred to as “varnish” or “gum”. This substance can block the small passages and orifices in the carburetor that are critical to properly metering the air/fuel charge. As a result of these types of blockages, the engine can run poorly or not at all. In addition, light components of the gasoline blend can tend to evaporate, leaving behind a fuel that has a very low vapor pressure, which again can result in the engine being very difficult or impossible to start. Further still, residual fuel in the carburetor can also result in corrosion in the carburetor due to ethanol and water content in the fuel. This corrosion can have the same effect as gum or varnish, blocking or restricting important passages in the carburetor.
The problem of residual fuel in the carburetor can be attributed in many cases to the fact that most single-cylinder, general-purpose gasoline engines currently use a float carburetor to meter and supply the air-fuel charge to the engine. Typically, the fuel delivery to the carburetor is regulated by a float-actuated needle valve in the carburetor, which maintains the fuel level in the carburetor during operation. When the unit is stored, this valve also continues to maintain the fuel level, supplying additional fuel from the fuel tank as fuel evaporates from the carburetor bowl. This additional supply can be stopped using some form of fuel shutoff valve, which can in some cases be linked to the engine shutoff.
Even with a fuel shutoff, however, there is often a significant amount of residual fuel remaining in the carburetor when an engine is put into storage, which can lead to many fuel system problems. To address this problem, some manufacturers include a drain in the carburetor, which allows the user to drain the carburetor in preparation for storage. In typical systems, however, a screwdriver, wrench, or other implement is required to open this drain. Additionally, many small engine operators do not understand the importance of draining the fuel, and therefore they do not drain the fuel from the carburetor before storing the engine.
Accordingly, it would be desirable for residual fuel remaining in the carburetor after engine shutoff to be automatically (or at least easily) drained so as to avoid the problems that can be caused by such residual fuel.